(1) Field of the Invention
This invention relates to a method of forming a mask which provides good critical dimension control, low defect levels, and maintains good throughput.
(2) Description of the Related Art
Wet isotropic etching and dry anisotropic etching are important processes for mask fabrication. Wet isotropic etching has the advantage of increased throughput while dry anisotropic etching has the advantage of improved critical dimension control.
U.S. Pat. No. 4,774,164 to Peavey et al. describes patterning a chrome mask with polybutenesulfone, PBS, photoresist. The photoresist is patterned and covered with glass. The glass is etched down to the thickness of the photoresist transferring the pattern to the glass. The photoresist is then removed and the remaining glass is used as a mask for etching the pattern in the chrome mask.
U.S. Pat. No. 5,738,337 to Tzu et al. describes a process for forming an attenuated phase-shifting photomask. Multiple exposure doses are used to expose a layer of resist material in forming the photomask.
U.S. Pat. No. 6,093,507 to Tzu describes a simplified process for fabricating Levinson and chromeless type phase shifting masks.
As critical dimensions in integrated circuit fabrication become smaller, such as in the range of 0.13 to 0.15 microns, dry anisotropic etching in the fabrication of chrome masks becomes necessary. While the dry anisotropic etching in the fabrication of chrome masks maintains the critical dimension tolerance, the process has low throughput and is subject to high defect levels when relatively large areas of chrome are etched away. While the defect levels for etching relatively large areas of chrome can be controlled using isotropic wet etching of chrome, the isotropic nature of the wet etching can not maintain adequate critical dimension control.
It is a principle objective of this invention to provide a method of fabrication of masks which combines the advantages of wet isotropic etching and dry anisotropic etching to maintain good critical dimension control, adequate throughput, and good defect levels for etching of relatively large areas of opaque material.
This objective is achieved by forming a frame around the pattern elements which require good critical dimension control. The opaque material, such as chrome, in this frame is then etched away using dry anisotropic etching. The remainder of the opaque material to be removed is then etched away using wet isotropic etching. This maintains good critical dimension control at the edges of the pattern elements, good throughput in mask fabrication, and good defect level control in removing the relatively large areas of opaque material which do not affect critical dimension control.